Methods and apparatus for R-wave detection

ABSTRACT

Apparatus and methods for determining the precise times of occurrence of R-waves in an ECG signal of a patient. Samples of the patient&#39;s ECG signal are accumulated and an ECG window comprising a predetermined number of the samples is defined. In accordance with one embodiment of the invention, the patient&#39;s blood pressure signal is processed to detect systoles and the ECG window has a predetermined time relationship to detected systoles. In accordance with an alternative embodiment, the ECG signal is first processed to approximate the times of occurrence of R-waves and the ECG window is defined to have a predetermined time relationship to the approximated R-wave times. ECG signal samples within the ECG window are processed to determine a maximum and the time of occurrence of the maximum is stored as the time of an R-wave event.

This application is a divisional of U.S. patent application Ser. No.08/940,396, filed Oct. 1, 1997 now U.S. Pat. No. 5,984,954.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENTS REGARDING FEDERALLY SPONSORED RESEARCH

Not applicable.

BACKGROUND OF THE INVENTION

An electrocardiogram (ECG or, sometimes, EKG) signal is an electricalsignal indicative of changes in electrical potential during a heartbeat,or contraction of the heart. ECG signals include features correspondingto certain aspects of each heartbeat and are used to diagnose cardiacactivity abnormalities. One such feature is the R-wave which ismanifested as a positive voltage spike in the ECG signal coincident witha ventricular contraction.

Various medical diagnostic and treatment apparatus require detection ofthe time of occurrence of the R-waves of a patient's ECG signal,including heart rate monitors and heart rate variability monitors. Heartrate variability monitors refer to a class of instruments with whichchanges in heartbeat rate are measured.

Conventional R-wave detection is typically achieved with a thresholddetection scheme by which the ECG signal is compared to a thresholdvalue, with the occurrence of the ECG signal exceeding the thresholdindicating the occurrence of an R-wave. R-wave detection is complicatedby the inherent noise and other artifacts typically associated with anECG signal. For this reason, the ECG signal is typically filtered priorto comparison with the threshold value.

The threshold detection method of detecting R-waves is often suitablefor use in heart rate monitors, since such monitors generally provide agross measure of heart rate which is subject to drift. However,threshold detection for detecting R-waves is less satisfactory for usein heart rate variability monitors, since detection of precise R-wavetimes is critical to providing an accurate heart rate variabilityindication.

In one conventional heart rate variability monitor, sold by BostonMedical Technologies under the product name HRView™ versions I and II,R-wave detection is achieved with a combination of filtering, thresholddetection and operator interaction. Once "approximate" R-wave locationsare detected by threshold detection, the operator views the ECG signaland approximate R-wave locations on a display and is able to removeR-wave detections which are likely to be false based on some criteria,such as the detections being too close in time to other detections. Theuser is also able to view a heart rate variability signal (i.e., heartrate versus time) generated in response to the detected R-waves andremove any heart rate variations which are likely to be false based onsome criteria, such as the magnitude of the variation. This technique isreferred to as "splining" since, in response to a user's indication thata particular heart rate variation exceeds some criteria, the heart ratevariability signal is "splined," so as to bridge the signal across theparticular heart rate variation.

It would be desirable to provide apparatus and techniques for detectingR-wave events with improved accuracy, particularly for use in heart ratevariability monitors which rely heavily on precise R-wave detection.

BRIEF SUMMARY OF THE INVENTION

The invention relates to methods and apparatus for detecting the timesof occurrence of R-wave events of a patient's ECG signal in response tothe patient's blood pressure signal and/or the patient's ECG signal. Ingeneral, an ECG window of the ECG signal in which an R-wave event isexpected to occur is defined for processing and the maximum signalsample within the ECG window is determined and stored as the precisetime of an R-wave.

In a first processing stage, the times of occurrence of R-waves of theECG signal are approximated. Thereafter, in a second processing stage,the approximate R-wave times are used to define the ECG windows whichare then processed to determine the precise R-wave times. With thisarrangement, the definition and precise processing of signal sampleswithin the relatively narrow ECG windows in which the R-wave eventsoccur makes precise R-wave detection possible, since such preciseprocessing of the entire ECG signal is computationally intensive andinfeasible.

In one embodiment, the R-waves of the ECG signal are detected inresponse to both the ECG signal and the blood pressure signal. Moreparticularly, the blood pressure signal is processed to determine thetime of occurrence of systoles, and the ECG window is defined to have apredetermined time relationship to the detected systoles.

In an alternative embodiment, the R-waves of the ECG signal are detectedin response to only the ECG signal which is initially processed byfiltering and threshold detection in order to determine the approximatetimes of occurrence of the R-waves. The ECG window is defined to have apredetermined time relationship to the approximate R-wave times.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of this invention, as well as the inventionitself, may be more fully understood from the following description ofthe drawings in which:

FIG. 1 is a block diagram of a monitor including features of theinvention;

FIG. 2 is a block diagram of a computer-based implementation of themonitor of FIG. 1;

FIG. 3 illustrates typical ECG and blood pressure signals;

FIG. 4 is a block diagram of the overall process of R-wave detection inresponse to an ECG signal and a blood pressure signal according to theinvention;

FIG. 5 is a flow diagram illustrating a method of implementing the firststage of R-wave detection of FIG. 4;

FIG. 6 is a flow diagram illustrating a method of implementing thesecond stage of R-wave detection of FIG. 4;

FIG. 7 is a block diagram of an alternative process of R-wave detectionin response to an ECG signal according to the invention;

FIG. 8 is a flow diagram illustrating a method of implementing the firststage of R-wave detection of FIG. 7; and

FIG. 9 is a flow diagram illustrating a method of implementing thesecond stage of R-wave detection of FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a block diagram of a monitor 10 includes aprocessor 14 for detecting the location of the R-waves of a patient'sECG signal, a memory 18, a display 22 and a data acquisition unit 26.The R-wave detection apparatus and techniques described and claimedherein are particularly well-suited for use in heart rate variabilitymonitors, which rely on precise R-wave detection. More generally,however, the monitor 10 detects the location of R-waves of a patient'sECG signal and it will be appreciated by those of ordinary skill in theart that the apparatus and techniques described herein are suitable foruse with other types of medical monitors.

The data acquisition unit 26 includes an ECG monitor 28 and, optionally,may further include a blood pressure monitor 30. The ECG monitor 28operates with a conventional ECG patient interface 38, such as electrodepads adapted for attachment to a patient's chest, and includes signalprocessing circuitry for conditioning the measured ECG signal forfurther processing. One suitable commercially available ECG monitor isof the type sold by Serena Medical Electronics Co., Inc. of San Jose,Calif. under the product name ECG Isolation Amplifier Module ModelECG-170. The output of the ECG monitor 28 is an amplified ECG signal 82which is converted into a digital signal 32 by an analog-to-digitalconverter 44.

Like the ECG monitor, the optional blood pressure monitor 30 is aconventional unit which is operative with a blood pressure patientinterface 40, such as a blood pressure cuff or a pulse oximetry-typedevice located near an atrial line at various locations on the patient'sbody, such as near the heart, a finger or earlobe. The blood pressuremonitor 30 includes signal processing circuitry, such as an amplifierand filter. The output of the blood pressure monitor 72 is an analogsignal which is converted into a digital signal 32 by theanalog-to-digital converter 44. One suitable commercially availableblood pressure monitor is of the type sold by Colin Medical InstumentsCorp. of San Antonio, Tex. under the product name Colin BP-508.

The digitized ECG and blood pressure signal 32 is coupled to theprocessor 14 for storage in the memory 18 via signal bus 34. While theillustrative monitor 10 stores digitized versions of the measured ECGand blood pressure signals (i.e., stores samples of the measured ECG andblood pressure signals), it will be appreciated by those of ordinaryskill in the art that the analog ECG and blood pressure signalsthemselves may alternatively be stored in memory 18 for processing,including subsequent analog-to-digital conversion.

The processor 14 may take various forms, such as the microprocessor of astandard personal computer, workstation or other microprocessor drivendevice. As one example, the processor 14 is an INTEL-compatiblemicroprocessor of an IBM-compatible personal computer running theMICROSOFT WINDOWS graphical user interface, The memory 18 includes aRandom Access Memory (RAM) and the user interface 16 may include akeyboard, touch screen and/or mouse. In the illustrative embodiment, theuser interface 16 includes a touch screen incorporated into the display22, the display is a flat panel LCD display of the type sold by Goldstarand the processor 14 and memory 18 are typical components associatedwith an IBM-compatible personal computer. It will be appreciated bythose of ordinary skill in the art that the apparatus and techniques ofthe system 10 may be implemented on various equipment, both hardware andsoftware.

In applications in which the monitor 10 is a heart rate variabilitymonitor 10, the processor provides additional processing includingpreparing the digitized signals for further time domain and spectralanalysis and interfacing with the display 22 via signal line, or bus 36to cause various data and information to be displayed to the user,including the stored ECG and blood pressure signals. Further, thedetected R-wave times are used by the processor 14 to generate a heartrate variability versus time signal which is likewise displayed to theuser.

Referring to FIG. 2, the ECG monitor 28 and/or blood pressure monitor 30may be implemented in the form of a "circuit module" adapted forinsertion into an Input/Output (I/O) port or slot 21 of a standardpersonal computer chassis 20. To this end, the circuit modules 28, 30are housed in a metal or metalized-plastic box, or instrument chassis24, which is adapted for insertion into an opening of the computerchassis 20 typically used for a diskette drive, tape drive or CD-ROMdrive.

The monitor 10 may include additional circuitry (not shown) formeasuring various other physiological phenomena of the patient, such asinspiration volume, in which case such circuitry is implemented in theform of one or more additional circuit modules. The instrument chassis24 has an open front face which permits insertion of one or more circuitmodule(s) therein. The front face of any unused portions of theinstrument chassis is covered by metal or metalized panels.Advantageously, the instrument chassis serves as a "Gauss cage" toreduce electromagnetic interference (EMI).

With this arrangement, the processor 14 can communicate with and controlthe ECG monitor module 28 and the blood pressure monitor module 30 viathe computer's standard busses and the circuit modules can draw on thepower available to the computer components. The processor 14 transmitsany control information to the circuit modules 28, 30 via the computer'sdigital control bus 42 and receives the digital ECG and blood pressuresignal 32 (FIG. 1) via the computer's digital control bus 42. Inapplications in which the circuit modules 28, 30 do not includeanalog-to-digital converters, an analog-to-digital converter 54 may beprovided to digitize the ECG and blood pressure signal. For safetyreasons, it is preferable to isolate the computer bus 42 from thecircuit modules 28, 30, for example, with opto-isolators 46, 48,respectively.

FIG. 3 illustrates a typical ECG signal 50 and a typical blood pressuresignal 52, drawn to the same time scale. R-wave occurrences are labeled56 on the ECG signal 50 and, on the blood pressure signal 52, systolesare labeled 58 and diastoles are labeled 60. It will be appreciated bythose of ordinary skill in the art that each diastole 60 precedes anassociated systole 58. The time scale is in seconds, with a typicalduration of a blood pressure pulse (i.e., the duration between time t₄and t₇) being approximately 0.5 seconds and a typical duration of anR-wave event 56 being on the order of 0.1 seconds.

As is apparent from the illustrative waveforms of FIG. 3, each R-waveevent 56 precedes a blood pressure systole 58 by between approximately50 and 200 milliseconds. As is further apparent from the waveforms, theattributes of the ECG signal 50 and the blood pressure signal 52 areperiodic. However, the period of the features associated with thesesignals can and does vary in accordance with static attributes of thepatient (e.g., the patient's weight) and dynamic attributes of thepatient (e.g., physical exertion of the patient).

Referring to FIG. 4, a diagram illustrating one R-wave detectiontechnique according to the invention is shown. In a first level, orstage 70 of R-wave detection, approximate times of systoles in thedigitized blood pressure signal 72 are determined. A second level 80 ofR-wave detection is responsive to the digitized ECG signal 82 forproviding data 92 indicative of the precise time of occurrence ofR-waves in the patient's ECG signal 82.

The technique illustrated by FIG. 4 will be described in greater detailin conjunction with the flow diagrams of FIGS. 5 and 6. In general, theblood pressure signal 72 is stored in a memory buffer as illustrated bystep 78. In step 84, the memory is accessed and a maximum signal levelwithin a search window is detected. The detected maximum within aplurality of search windows comprise the systole times as represented byoutput 74.

In accordance with the second level 80 of R-wave detection, the ECGsignal 82 provided by monitor 28 is filtered in step 86, such as with alowpass filter. The filtered ECG signal is then stored in a memorybuffer in step 88. In step 90, an ECG window of the ECG signal isdefined and searched in order to determine its maximum. It is thesemaxima within a plurality of ECG windows that collectively comprise theprecise times of R-waves of the patient's ECG signal as represented byoutput 92.

FIG. 5 illustrates one technique for implementing the first stage 70 ofR-wave detection of FIG. 4. The process commences in step 100, followingwhich a blood pressure signal sample and an ECG signal sample areacquired by the data acquisition unit 26 (FIG. 1) in step 104. Thesample acquisition step 104 may be performed at a predeterminedfrequency, such as on the order of once every millisecond. The ECGsignal sample and the blood pressure signal sample are, preferably,simultaneously acquired.

The sample acquisition step 104 is repeated until it is determined instep 108 that a predetermined number of blood pressure samples has beenacquired. The predetermined number of samples represents a "large searchwindow" and the time interval represented by the large search window isselected to break down the task of processing of the blood pressuresignal in order to achieve signal processing efficiencies while insuringthat only a single systole occurs in each such window. In theillustrative embodiment, the large search window represents a timeinterval approximately equal to the duration of a typical blood pressurepulse (i.e., from time t₄ -t₇ in FIG. 3), such as 0.5 seconds. The largesearch window interval may be a predefined time interval or,alternatively, may be user selectable and/or adjustable. Moreparticularly, the user may select the time interval of the large searchwindow or the user may input certain information about the patient, suchas the patient's body weight, in response to which the large searchwindow interval is computed.

Once a sufficient number of blood pressure signal samples to constitutea large search window have been acquired, a blood pressure signal sampleis selected for processing in step 112. As will become apparent, thesamples within the large search window are processed to, initially, finda maximum (step 116). During the first iteration of the process, thesample selection defines a point in the middle of the large searchwindow, such that there is sufficient data in memory before and afterthe point to determine if the point is a systole. During subsequentiterations of the process, the selected sample is the sample followingthe sample selected during the previous iteration.

Various schemes may be used to select the sample to be processed duringthe first iteration of the process. In accordance with one method ofselecting the first sample for processing, the rise time of a bloodpressure pulse, either measured or typical, is used. The rise timedetermines how far back signal samples must be processed in the eventthat the selected sample represents a systole. Stated differently, thefirst sample selected for processing occurs after a time interval equalto at least the rise time of the blood pressure pulse has lapsed.

The blood pressure pulse rise time information may be provided to theprocessor 14 in various ways. As one example, the user views the bloodpressure signal on the display 22, visually determines the rise time andenters this information via the user interface 16. Thus, taking theblood pressure signal 52 (FIG. 2) as an example of what might bedisplayed, the user viewing the first pulse notes that its rise time ison the order of (t₅ -t₄). Upon entering this information into themonitor 10, the first sample selected for processing in step 112 occursat time (t₅ -t₄). Alternatively, the typical rise time may be apreselected, default value or may be user adjustable and/or selectable,such as by having the user enter certain information regarding thepatient, such as the patient's age and/or body weight.

In step 116, it is determined whether the selected sample is the maximumwithin a "small search window." Samples within a small search window areprocessed together in order to enhance the efficiency of signalprocessing. The selection of the time interval represented by the smallsearch window is a function of many factors, including the speed of theprocessor 14 and the frequency content of the signal (i.e., the morehigh-frequency noise associated with the signal, the larger the optimumsmall search window, in order to reduce the likelihood of falsedetections). In the illustrative embodiment, the width of the smallsearch window is on the order of 0.1 seconds. The time intervalrepresented by the small search window may be preset or, alternatively,may be user selectable and/or adjustable.

Step 116 comprises multiple iterations during each iteration of theoverall process of FIG. 5. The way in which it is determined whether theselected sample is a maximum in the small search window is by comparingits amplitude to that of all samples within the same window. Moreparticularly, upon the first iteration within step 116, when theselected sample in the small search window is being processed, itsamplitude is compared with the first value in the small window todetermine which is greater. During the second iteration within step 116,the selected sample in the small search window is compared with thesecond value in the small window to determine which is greater.

In the event that the selected sample is not greater or equal to everysample it is compared to in the small search window, then the process isrepeated starting with step 104, as shown. That is, if the processedsample is not a maximum in the small window, then the next sample isacquired, etc. Alternatively, if it is determined in step 116 that theselected sample is the maximum in the small search window, then the timeof occurrence and amplitude of the processed sample is stored as the newmaximum within the window.

In subsequent step 120, it is determined whether the blood pressuresignal fell by at least a predetermined amount during an intervalpreceding the occurrence of the maximum. In the event that the bloodpressure signal did fall by at least the predetermined amount during theinterval preceding the maximum, then step 124 is performed in which itis determined whether the blood pressure signal fell by at least apredetermined amount during an interval following the maximum. If bothof these queries are answered in the affirmative, then a systole hasbeen located and, in subsequent step 128, the time of the selectedsample is stored as that of a systole. Alternatively, if either theblood pressure signal did not fall by at least the predetermined amountduring the interval preceding the maximum, or the blood pressure signaldid not fall by at least a predetermined amount during the intervalfollowing the maximum, then the process is repeated starting at step104, as shown.

The purpose of steps 120 and 124 is to determine whether the particularlarge search window in which the selected sample occurs contains asystole. For example, consider the case where the large search windowextends from time t₅ -t₇ and the sample at time t₆ has been selected forprocessing in step 112. Processing a small search window centered attime t₆ in step 116 reveals that the sample occurring at time t₆ is themaximum within that small search window. However, it will subsequentlybe determined in step 120 that the blood pressure signal did not fall bya predetermined amount during an interval preceding the maximum. Thisdetermination will cause the process to be repeated since the maximum attime t₆ does not represent a systole.

The predetermined amounts used in steps 120 and 124 may be predefinedvalues, or alternatively, may be user selectable and in both cases, areselected (either by the manufacturer or the user) in order todistinguish a systole from other artifacts in the blood pressure signal.In the illustrative embodiment, the predetermined amount is a value thatis selected by the user, on the order of 5 mmHg. Further, thepredetermined amounts of steps 120 and 124 may be the same or,alternatively, may be different. In applications in which thepredetermined amounts of steps 120 and 124 are user selectable, theselection may be based on the user viewing the blood pressure signal anddetermining a value suitable for distinguishing a systole from otherartifacts or, alternatively, may be based on attributes of the patientwhich are entered by the user, such as the patient's weight and/or age.

Also stored in step 128, in association with the time of occurrence ofeach systole, is the amplitude of the systole, the time of the diastoleassociated with the systole, the amplitude of the diastole and theaverage blood pressure for the particular beat. In determining the timeof the diastole associated with a stored systole, signal samplespreceding the time of each stored systole are processed. In oneembodiment, the time interval in which the diastole is searched is equalto the rise time of the blood pressure pulse which is provided to theprocessor 14 in step 112 (FIG. 5). The signal samples preceding adetected systole are processed to detect a minimum until the signalbegins to climb again. The average blood pressure for the beat isdetermined by integrating the blood pressure signal over the timeinterval between two diastoles. Thereafter, the process terminates instep 130. However, the entire process of FIG. 5 is repeated to find eachsystole/diastole pair in a collected data sample.

The second stage 80 of R-wave detection according to the method of FIG.4 is shown in FIG. 6. Recall that this stage of processing is responsiveto the digitized ECG signal 82 as well as to the stored systole timesgenerated in the first stage 70 of processing. The second stage 80 ofprocessing commences in step 140, following which an ECG window isdefined in step 144. The ECG window is a time interval preceding adetected systole time. Thus, this step includes the processor 14accessing a stored systole time from memory 18 and computing a timeinterval preceding the stored time. The ECG window represents a timeinterval during which an R-wave event 56 (FIG. 3) is expected to haveoccurred and is the interval during which ECG signal samples will beprocessed to locate an R-wave event. The ECG window thus, commences at apredetermined time prior to the stored systole time and terminateseither some time before the stored systole time or at the stored systoletime. In the illustrative embodiment, the ECG window commences 250milliseconds preceding a stored systole time and terminates 25milliseconds before the stored systole time. Further, the ECG window maybe a predefined window (i.e., having a predefined, or default durationand time relationship to a detected systole) or, alternatively, may beuser selectable and/or adjustable. As one example, the ECG windowduration and time relationship to a systole is computed by the processor14 in response to certain patient information, such as the patient'ssize and/or the location from which the blood pressure signal isacquired (e.g., the finger or arm), which is input by the user via theuser interface 16.

Once the ECG window is defined in step 144, the first sample from theECG window is selected for processing in step 148. Subsequently, in step152, it is determined whether the selected ECG signal is the maximum ofthe samples in the ECG window. This determination is performed in aniterative manner similar to the maximum determination in step 116 ofFIG. 5. That is, the amplitude of the selected sample is compared to thepreviously processed maximum amplitude within the window to determinewhether the selected sample represents a "new" maximum. In the eventthat the selected sample is not the maximum in the ECG window, then step160 is next performed, as shown. Alternatively, if the selected sampleis the maximum in the ECG window, then the time of the selected sampleis stored as a possible R-wave event time in step 156. However, it isnot until all samples in the ECG window are processed that adetermination can be made as to whether the possible R-wave event timerepresents an actual R-wave event time. Thus, in subsequent step 160, itis determined whether the processed sample is the last sample in the ECGwindow. If this is not the case, then step 162 is performed in which thenext ECG signal sample from the ECG window is selected for processing,following which the process is repeated starting at step 152 until thelast sample has been processed. Once the last sample has been selectedfor processing, then the time of the selected sample representing themaximum in the ECG window is stored as the time of an R-wave event instep 164. Thereafter, the process terminates in step 168.

With the above method and apparatus, the precise times of R-wave eventsare determined from digitized ECG and blood pressure signals to aprecision determined by the data sampling rate, for example, to withinapproximately 0.001 seconds for a data sampling rate of 0.001 secondsper sample. The first stage of processing (FIG. 5) is used to determinethe times of systoles within the blood pressure signal and, thisinformation is used in the second stage of processing (FIG. 6) to definean ECG window of the ECG signal during which the R-wave event occurs.Having defined the ECG window, samples of the ECG signal within the ECGwindow are processed in a manner that ensures that the precise R-waveevent time is determined.

Referring to FIG. 7, an alternative R-wave detection scheme isillustrated. The R-wave detection of FIG. 7 is achieved without use ofthe patient's blood pressure signal and is responsive only to the ECGsignal 82 provided by the ECG monitor 28 (FIG. 1). The R-wave detectionof FIG. 7 includes two stages of processing 170, 172. In accordance withthe first stage 170, the digitized ECG signal 82 is filtered by abandpass filter 174 to provide a filtered ECG signal 178. Since thefeature of interest in the ECG signal is the R-wave event, which istypically accompanied by frequency content on the order of 17 Hz+/-3 Hz,in the illustrative embodiment, the bandpass filter 174 has a nominalcenter frequency of 17.0 Hz and a pass band of approximately 6 Hz.

The absolute value of the filtered ECG signal 178 is calculated in step184. By taking the absolute value of the filtered ECG signal, all of thestored samples of the ECG signal are positive. The positive-going pulses188 may, optionally, be lowpass filtered in step 190, as shown, in orderto attenuate noise due to brief episodes of 17 Hz noise to provide afurther filtered signal 192.

In a process step 194, each of the ECG signal samples is compared to athreshold level. If a processed signal sample exceeds the thresholdlevel, then the time of that sample is stored as an approximate R-wavetime. Alternatively, the time of the sample is not stored. Thus, theoutput of the comparison step 194 is a set 196 of stored times of signalsamples which correspond to approximate R-wave times. The thresholdlevel used in step 194 may be a predetermined value and/or may have apredefined relationship to "recent" previously processed signal samples.In one embodiment, the threshold level is a predetermined percentage ofthe average peak values of recent signal samples. Before a sufficientnumber of signal samples are processed to determine the threshold level,it may be desirable to store a short period of lowpass filtered ECGsignal samples processed by steps 174 to 190 and set the threshold levelto a percentage of the average peak values of the signal 192 during thatperiod.

The second stage 172 of R-wave detection relies on the approximateR-wave times provided at the output 196 of the first processing stage170 and, further, on the digitized ECG signal 82. The digitized ECGsignal 82 is lowpass filtered in a step 200, preferably by a filter thatpreserves time-domain content of the signal, such as a filter having an"elliptical" response. The filtered signal samples 202 are stored inmemory in step 206. In a process step 210, the approximate R-wave times196 provided by the first stage 170 of R-wave detection are used todefine an ECG window within the filtered signal portion stored in step206 for processing to determine the precise times of R-waves. Moreparticularly, an ECG window is defined to have a predetermined timerelationship with respect to each approximate R-wave time and apredetermined duration. The stored samples are accessed and processed ingroups defined by ECG windows which have a predetermined timerelationship with respect to the approximate R-wave times determined instage 170. The maximum signal sample within each ECG window yields aprecise R-wave time at the output 214 of the second stage 172 of R-wavedetection.

The process of FIG. 7 is described in more detail in conjunction withthe flow diagrams of FIGS. 8 and 9. FIG. 8 illustrates a method ofimplementing the first stage 170 of R-wave detection and FIG. 9illustrates a method of implementing the second stage 172 of R-wavedetection.

Referring to FIG. 8, the process commences in step 220, following whichan ECG signal sample is acquired by the data acquisition unit 26(FIG. 1) in step 224. In step 226, the acquired ECG signal sample isstored in a raw data array. Thereafter, in step 228, the stored ECGsignal sample is bandpass filtered. Various apparatus and methods aresuitable for filtering the ECG signal sample in step 228, including asoftware filter.

In subsequent step 232, the absolute value of the processed signalsample is computed, to ensure that all pulses are positive-going.Optionally, the sample may be lowpass filtered in step 234.Subsequently, in step 236, the processed signal sample is stored in aprocessed data array.

In process step 238, it is determined whether a predetermined number ofECG signal samples have been acquired. The predetermined number isassociated with an ECG signal processing window of sufficient width tocontain an R-wave event. In the illustrative embodiment, thepredetermined number of samples are those acquired within an interval onthe order of 0.2 seconds. In the event that the predetermined number ofsamples has not been acquired, then it is determined in step 250 whetherdata collection has been completed. If data collection has beencompleted, then the process terminates in step 256. Alternatively, theprocess is repeated, starting with acquisition of another ECG signalsample in step 224, as shown.

If it is determined in step 238 that enough signal samples have beenacquired to comprise an ECG search window, then the next sample in theECG window is selected for processing in step 240 and, it is nextdetermined in process step 242 whether the selected sample exceeds athreshold level. In the illustrative embodiment, the selected sampleprecedes the most recently acquired sample by a predetermined time. Aswill become apparent, each processed sample is compared to the thresholdlevel to determine whether the sample exceeds the threshold and thus, isindicative of an approximate R-wave time. The threshold level may be apredefined, or predetermined level, it may be a function of theamplitude of certain ECG signal samples or it may be user selectable oradjustable. In the illustrative embodiment, the threshold level is apredetermined percentage of the average peak values of a predeterminednumber of recently processed (steps 224 through 236) ECG signal samples.A suitable number of ECG signal samples from which to compute thethreshold value is on the order of five or more and the predeterminedpercentage of the average peak values is on the order of 30%.

Prior to a certain number of iterations of the process of FIG. 8,however, the predetermined number of ECG signal samples on which to basethe threshold level will not yet have been processed. During this time,a predetermined threshold level may be used or processing may be delayeduntil after the predetermined number of samples have been accumulated.

In the event that it is determined in step 242 that the selected signalsample is not greater than the threshold level, then step 250 is nextperformed, as shown. Alternatively, it is next determined in step 244whether a lockout time interval has lapsed since the last detection of aselected sample exceeding the threshold. The lockout time interval isselected to prevent noise from causing false R-wave detections. In theillustrative embodiment, the lockout time interval is on the order of0.2 seconds. In the event that the lockout time interval has not lapsed,then the process is repeated starting at step 250. Alternatively, thetime of occurrence of the selected sample is stored in step 248 as theapproximate time of an R-wave event, following which step 250 isrepeated.

Referring to FIG. 9, the second stage 172 of R-wave detection accordingto the process of FIG. 7 commences in step 260, following which an ECGwindow is defined in step 262. The ECG window is a portion of thedigitized and stored ECG signal during which an R-wave event is expectedto occur. In the illustrative embodiment, the ECG window has apredefined time relationship centered around an approximate R-wavelocation stored in step 248 of FIG. 8 and a duration on the order of0.15 seconds. It will be appreciated by those of ordinary skill in theart however, that more generally, the ECG window may be defined in othermanners and, in fact, may be user selectable in terms of its durationand relationship to an approximate R-wave time stored in step 248.

In subsequent step 264, the first sample of the digitized ECG signal inthe ECG window is selected for processing. Thereafter, in step 266, itis determined whether the selected sample is the maximum of the samplesthat have been processed within the ECG window. As discussed previously,this determination is made by comparing the selected sample with thepreviously processed signal sample having the greatest amplitude in theECG window.

In the event that the selected signal sample is not the maximum withinthe ECG window, then step 270 is next performed in which it isdetermined whether the selected sample is the last sample of the ECGwindow. Alternatively, the time of the selected sample is stored in step268 as the time of a possible R-wave event. In step 270, if it isdetermined that the selected sample is not the last sample within theECG window, then step 272 is performed in which the next ECG sample fromthe ECG window is selected for processing, following which the processis repeated starting at step 266, as shown. Alternatively, the time ofthe selected sample is stored as the time of an R-wave event in step274, following which the process terminates in step 276.

As will now be apparent, the embodiments of the present inventioninclude defining a relatively narrow ECG window of the ECG signal inwhich the R-wave events occur. The samples within the defined ECGwindows are processed in a manner which provides a precise determinationof R-wave events. The precision of the R-wave event detection is madepossible by the definition and precise processing within the relativelynarrow ECG windows in which R-wave events occur, since such preciseprocessing of the entire ECG signal is computationally intensive andmore liable to be affected by artifacts, such as noise, lead movement,muscle movement, etc.

Having described the preferred embodiments of the invention, it will nowbecome apparent to one of ordinary skill in the art that otherembodiments incorporating their concepts may be used. It is felttherefore that these embodiments should not be limited to disclosedembodiments but rather should be limited only by the spirit and scope ofthe appended claims. All publications and references cited herein areexpressly incorporated herein by reference in their entirety.

What is claimed is:
 1. A method of detecting a plurality of R-waves ofan ECG signal of a patient, comprising the steps of:storing the ECGsignal of the patient; storing a blood pressure signal of the patient;and detecting the R-waves of the ECG signal of the patient in responseto the stored ECG signal and the stored blood pressure signal.
 2. Themethod of claim 1 wherein the R-wave detecting step includes:detectingapproximate times of the R-waves of the ECG signal in response to thestored blood pressure signal; and detecting precise times of the R-wavesof the ECG signal in response to the stored ECG signal and theapproximate times of the R-waves.
 3. The method of claim 2 wherein theapproximate R-wave times detecting step includes:searching apredetermined portion of the stored blood pressure signal for a maximum;determining if the stored blood pressure signal fell by a firstpredetermined amount during a first interval preceding the maximum;determining if the stored blood pressure signal fell by a secondpredetermined amount during a second interval following the maximum; andstoring the time of the maximum of the stored blood pressure signal asan approximate R-wave time if the stored blood pressure signal fell bythe first predetermined amount during the first interval preceding themaximum and fell by the second predetermined amount during the secondinterval following the maximum.
 4. The method of claim 3 wherein theprecise R-wave times detecting step includes:searching a predeterminedportion of the stored ECG signal preceding an approximate R-wave timefor a maximum; and storing the time of the maximum of the stored ECGsignal as a precise R-wave time.
 5. Apparatus for detecting a pluralityof R-waves of an ECG signal of a patient comprising:a memory for storingthe ECG signal of the patient and a blood pressure signal of thepatient; and a processor responsive to the stored ECG signal and thestored blood pressure signal for detecting the R-waves of the ECGsignal.
 6. The apparatus of claim 5 wherein the processor includes afirst processor portion for detecting approximate times of the R-wavesin response to the stored blood pressure signal and a second processorportion for detecting precise times of the R-waves in response to thestored ECG signal and the approximate times of the R-waves.
 7. Theapparatus of claim 6 wherein the first processor portion includes:adetector for detecting a maximum of the stored blood pressure signaloccurring within a predetermined window of the stored blood pressuresignal; and a comparator for comparing the blood pressure signal to afirst threshold to determine whether the blood pressure signal fell by afirst predetermined amount during a first interval preceding the maximumand for comparing the blood pressure signal to a second threshold todetermine whether the blood pressure signal fell by a secondpredetermined amount during a second interval following the maximum,wherein the time of the maximum is stored in the memory as anapproximate R-wave time if the stored blood pressure signal fell by thefirst predetermined amount during the first interval preceding themaximum and the stored blood pressure signal fell by the secondpredetermined amount during the second interval following the maximum.8. The apparatus of claim 7 wherein the second processor portionincludes a detector for determining a maximum of the stored ECG signaloccurring within a predetermined window of the stored ECG originalpreceding an approximate R-wave time, wherein the time of the maximum ofthe stored ECG signal is stored in the memory as a precise R-wave time.9. A method of detecting a plurality of R-waves of an ECG signal of apatient, comprising the steps of:storing the ECG signal of the patient;storing a blood pressure signal of the patient; detecting the R-waves ofthe ECG signal of the patient in response to the stored ECG signal andthe stored blood pressure signal, said detecting step including thesteps of:detecting approximate times of the R-waves of the ECG signal inresponse to the stored blood pressure signal; and detecting precisetimes of the R-waves of the ECG signal in response to the stored ECGsignal and the approximate times of the R-waves.
 10. Apparatus fordetecting a plurality of R-waves of an ECG signal of a patientcomprising:a memory for storing the ECG signal of the patient and ablood pressure signal of the patient; and a processor responsive to thestored ECG signal and the stored blood pressure signal for detecting theR-waves of the ECG signal, said processor including:a first processorportion for detecting approximate times of the R-waves in response tothe stored blood pressure signal; and a second processor portion fordetecting precise time of the R-waves in response to the stored ECGsignal and the approximate times of the R-waves.